use assert[Not]In where appropriate

A patch from Dave Malcolm.
diff --git a/Lib/test/test_posix.py b/Lib/test/test_posix.py
index 701d1ea..87c63ad 100644
--- a/Lib/test/test_posix.py
+++ b/Lib/test/test_posix.py
@@ -251,7 +251,7 @@
 
     def test_lsdir(self):
         if hasattr(posix, 'lsdir'):
-            self.assertTrue(support.TESTFN in posix.lsdir(os.curdir))
+            self.assertIn(support.TESTFN, posix.lsdir(os.curdir))
 
     def test_access(self):
         if hasattr(posix, 'access'):